I decided to switch to systemd (because of Gnome 3.8 and for other reasons). I followed the instructions on http://wiki.gentoo.org/wiki/Systemd carefully, but... my system does not boot at all. The boot process hangs after The messages

it turned out to be a problem with the initramfs created by Dracut. An initramfs created with genkernel works. Some problems remain, for example that the kernel does recognize my root partition if I use systemd as init system. But for this problem there is already another thread: https://forums.gentoo.org/viewtopic-t-965698.html